170 Plant Physiology 



criticism would be equally valid in many other lines of 

 investigation. 



91. The nature of the special roles. - - Certain soil ele- 

 ments are needed in the building up of the permanent pro- 

 teins of the living matter. Those which are known to 

 enter invariably into the composition of albuminoidal or 

 protein bodies are necessarily of first importance. Other 

 essential mineral elements play only doubtful roles in pro- 

 tein activities, yet they have evidently such important 

 functions to perform in connection with the activities of 

 the protoplasm and its products as to be indispensable. 



Practically, as expressed by Reed, we may say that in 

 general essential elements appear to function in two ways : 

 (a) as component parts of necessary cell structures and 

 fluids ; and (6) as agents indirectly essential, by causing 

 less understood physical or chemical reactions, - - acting 

 as carriers of other ions, as specific antidoting agents, or 

 otherwise. 



The first group includes, among the elements now under 

 discussion, nitrogen, phosphorus, and sulfur; while 

 potassium, calcium, magnesium, and iron fall apparently 

 in the second group. If the chemical work of the future 

 demonstrates fully the existence of the basic proteins, 

 now postulated, as noted later, it would then only, perhaps, 

 be safe to assume the incorporation of these elements into 

 the protoplasm itself. The latter elements (especially 

 potassium) may be important in the osmotic work of the 

 cell, requisite as carriers or accumulators of food atoms, 

 as catalytic agents, etc. ; but with nitrogenous bodies 

 like proteins they seem to form at most only temporary 

 combinations. 



